At 01:49 PM 7/11/2012, David Beierl wrote: >Open neutral between the breaker box and the service entrance is >much less common but more dangerous, and will NOT give the readings >I said above. It will give these instead: > >H-N 0-240 V; H-G 120 V; N-G 0-120 V. The closer N-G is to 0, the >closer H-N will be to 120. Voltages may change abruptly as loads >shift throughout the house/camp. > >Explanation to follow. US domestic wiring comes into the house on three wires, two hots and a neutral. Each hot wire is 120 VAC with respect to ground/neutral, but they are in opposite phase; in other words they cross zero volts at the same time, but then one goes negative while the other goes positive. As a result, though they each carry 120 V to ground, they carry 240 V between themselves. At the circuit breaker box, the breakers are mounted on a tree such that each successive breaker going down a column is energised by the other hot wire from its neighbors. That way, to get a 240 V circuit for stoves etc it's only necessary to mount a twinned breaker with two wires to the appliance. The other breakers each have one wire to the circuit they feed, and a return wire comes back to the neutral bus inside the box which is connected to the neutral service feed (which is at some point grounded). So in normal operation, half the 120 V circuits are out of phase with the other half, but nobody cares. Both phases return to the neutral bus which is grounded, and each appliance is connected between one of the phases and neutral, and sees 120 V. But if the neutral bus should be disconnected from the neutral service wire, everything changes. Now all the (operating) appliances on one phase are connected through all the ones on the other phase. The voltage each set sees is in inverse proportion to the amount of load on that phase. So if you've got let's say only a ten-amp toaster connected on one phase and only a 1/10-amp compact fluorescent light operating on the other phase, the toaster will be cold and the light will suddenly discover the thrills of operating at twice its rated voltage. Usually the imbalance won't be that extreme, but you get the idea. I've seen this happen once, after hours at an electronics company I worked for. I was one of two people in the place and we figured it out, but not in time to save a couple of smoking radios. Fortunately most of the fancy gear was turned off. Yours, David |
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